# Lesson Plan: Which Light Bulb is Really Cheaper (6-9)

*Adapted from “How Many Light Bulbs Does it Take to Change a People,” a Collaborative Project between the New England Electric System and the Conservation Law Foundation*

**Essential question**: What skills will students need to assess energy waste and savings in the school?

## Overview/Objective

Students will calculate and compare the long-term costs of incandescent bulbs versus energy-efficient fluorescent bulbs.

**Time**: 1 hour

**Subject:** Math

**Suggested Grade Level**: 5 – 9

**Materials:** Student worksheet (included)

## Background

Effective conservation efforts have two fronts: behavioral changes and changes in equipment. Behavioral changes include turning the lights off and keeping the thermostat several degrees lower. Equipment changes include replacing inefficient incandescent lights with more energy efficient fluorescent lights – then making those fluorescent lights even more efficient through such equipment as reflectors and high-efficiency ballasts. A 15-watt fluorescent bulb puts out roughly the same light as a 60-watt incandescent. With a reflector aimed at a specific point, a single 15-watt fluorescent bulb puts out more light than two 60-watt bulbs!

Incandescent bulbs work on the principle of electrical resistance. Current flows across a filament – a small strand of tungsten wire. That filament resists the flow of electrons more than the conductors (copper wire) that carry the electricity to it. As the electrons slow down, they heat up and begin to glow. In other words, incandescent bulbs give off both light and heat. Because their function is only to emit light, the electricity they use in making and giving off heat is wasted. Thus, they are inefficient.

Fluorescent bulbs, on the other hand, have no filament and do not work on the principle of electrical resistance. They use the flow of electrons (electricity) to excite gas atoms, which in turn fluoresce, or give off light. The inside of a fluorescent bulb, therefore, contains a gas rather than a filament. In addition, fluorescent fixtures contain an electrical device called ballast which stores and intensifies the current to provide something like a kick start to the gas inside the tube. That charging and intensifying of current is why fluorescent bulbs need a few seconds to go on once you throw the switch. Once they are lit, fluorescent bulbs emit a great deal of light and practically no heat. Thus, they are much more efficient than incandescent bulbs.

Which Light Bulb is Really Cheaper?

## Procedure

The effects of an energy conservation effort can be quite dramatic when presented numerically in terms of fuel savings, electricity savings, dollar savings, or environmental impact. The calculations in this activity require only simple arithmetic. This activity illustrates the value of conservation and provides good math practice.

**Part I: The Cost of an Incandescent Bulb**

An incandescent light bulb costs about 50 cents and lasts about 750 hours. If you were to leave a 100-watt bulb burning for 24 hours each day, 365 days per year, it would use 876 kilowatt hours per year. (1000 watts equals one kilowatt – “kilo” means a thousand.)

- 100 watts x 24 hours x 365 = 876,000 watt hours (Wh)
- 876,000 watt hours/ 1,000 = 876 kilowatt hours (kWh) (the power company bills us in kWh)

A kilowatt-hour of electricity costs about 15 cents. Your light bulb therefore costs about $131.40 per year to operate (876 kWh X 15 cents).

- 876 kWh x 0.15 cents = $131.40

In addition, it must be changed every 750 hours, so you must buy 12 bulbs each year.

- 24 hrs x 365 days = 8,760 hours per year/750 hours per light bulb = 11.68 or 12 light bulbs

If one incandescent light bulb costs 50 cents, you would have to pay $6.00 for bulbs each year.

- 12 bulbs x $0.50 per bulb = $6.00

**Part II: The Cost of A Fluorescent Bulb**

A high-efficiency fluorescent bulb costs about $2-$5 and lasts about 8,000 hours. A 25-watt fluorescent bulb puts out roughly the same amount of light as a 100-watt incandescent light.

If you were to leave a 100-watt bulb burning for 24 hours each day, 365 days per year, it would use 216 kilowatt hours per year. (1000 watts equals one kilowatt – “kilo” means a thousand.)

- 3. 25 watts x 24 hours x 365 = 216,000 watt hours (Wh)
- 216,000 watt hours/ 1,000 = 216 kilowatt hours (kWh) (the power company bills us in kWh)

The cost of a kilowatt-hour of electricity varies across the country, but average cost (including all charges) is about 15 cents. Your light bulb therefore costs about $32.40 per year to operate (216 kWh X 15 cents).

- 216 kWh x 0.15 cents = $32.40

In addition, it must be changed every 8,000 hours, so you must buy 2 bulbs each year.

- 24 hrs x 365 days = 8,760 hours per year/8,000 hours per light bulb = 1.095 or 2 light bulbs

If one incandescent light bulb costs $4.00, you would have to pay $8.00 for bulbs each year.

- 2 bulbs x $4.00 per bulb = $8.00

**Part III: Comparing the Cost of the Incandescent and Fluorescent Bulbs
Year I:**

At the end of the first year, the incandescent light bulb will have cost $131.40 to operate, and new bulbs will have cost $6.00.

What has the total cost of the incandescent bulb been?

- $131.40 +$ 6.00 = $137.40

The fluorescent bulb cost $32.40 to operate, and new bulbs will have cost $8.00.

What has the total cost of the fluorescent bulb been for the year?

- $32.40 + $8.00 = $40.40

Which light bulb is cheaper? The one that costs 50 cents of the one that costs $4?

## Extensions

Use this lesson as a spring board for surveying lighting usage in your school building.

Contact your local utility to see if they have inexpensive CFLs. Consider using the CFLs for a school fundraiser.